1. Field of the Invention
The present invention relates to a method and an apparatus for separating contaminants from paper stock and defibering undefibered waste paper in the field of industries using waste paper pulp as stock such as paper pulp and fiberboard industries.
Screening is generally composed of coarse and fine screening stages.
2. Discussion of the Background
In the coarse screening stage, relatively large contaminants are removed, using a screen plate with holes usable for relatively high consistency (2 to 4%) of stock slurry in order to reduce in quantity the contaminants to be transferred to the fine screening stage.
In the fine screening stage, fine contaminants not removable by the above-mentioned hole screen plate are removed, using a screen plate with slots suitable for relatively low consistency (0.5 to 2%) of stock slurry so as to facilitate passing of the stock through the screen.
Generally, the efficiency or ratio of removing contaminants in a screen is closely related with the rejection ratio. Increase and decrease of the rejection ratio lead to enhancement and lowering of contaminant removal ratio, respectively. Attempts to reduce the rejection ratio in an ordinary screen will tend to cause plugging of the screen plate or plugging of a rejection valve due to increased rejection consistency. Even if such plugging may be averted, an extreme reduction of the rejection ratio would worsen the effect of removing contaminants as shown in FIG. 1, thus failing to obtain good screening effect. Increase of the rejection ratio to a certain extent is therefore required for obtaining pulp with less quantity of contaminants. However, increase of the rejection ratio means reduction of yield.
Generally, in order to overcome this problem in a screen stage, a rejection ratio of 20 to 25% is selected, over which the curve shown in FIG. 1 becomes dull and the contaminant removal ratio is less affected, and rejected stock is reprocessed by a so-called "multiple cascade flow" system to reduce the rejection ratio in the entire system. In a typical cascade flow employed, rejected stock of a primary screen is processed by a secondary screen and the accepted stock is brought to the primary screen. Rejected stock of the secondary screen are processed by a tertiary screen and the accepted stock is returned to the feed stock of the secondary screen. Only rejections of the tertiary screen are discharged out of the system. Generally, stock slurry consistency in a screen becomes higher than the consistency of the feed stock and therefore the feed stock used for the cascade flow is required to be diluted with water into an appropriate consistency for the screen.
On the other hand, paper stock to be fed to a screening stage is in the form of defibered suspension of waste paper in water by a defibrator, usually called a pulper. Defibering performance of the pulper is not in a linear relationship to defibration time period (motive power). In comparison with initial defibering performance, subsequent defibering performance is decreased. That is, defibering efficiency is satisfactory up to a certain level of defibration [i.e., defibered stock/(defibered stock+undefibered stock)] and higher motive power is required for defibration over the level. In order to defiber the stock which has been defibered to the certain level, a device generally called "secondary defibrator" is widely used. Typical secondary defibrators are a closed pulper type defibrator and a high-speed defibrator. Such secondary a defibrators also have defibering performance which is not in a linear relationship to motive power and are effective for use at a zone or portion of the system where undefibered waste paper is accumulated.
To defiber undefibered waste paper is very significant for improvement of production yield since the undefibered waste paper shows the same behavior as contaminants to be removed in screening stages.
In FIG. 2 which is a flow sheet of a conventionally used screening process for waste paper stock pulp slurry, reference symbol a represents a tank to receive waste paper stock slurry which has been defibered by a pulper (not shown). In a coarse screening stage A, reference symbols b, c and d represent primary, secondary and tertiary coarse screening screens, using hole screen plates, respectively; g represents a high-speed defibrator for defibering rejections of the primary coarse screening; and e, f and m denote tanks. In a fine screening stage B, reference symbols h, i, k, and l represent primary, secondary, tertiary and quaternary fine screenings, using slot screens, respectively; j indicates a high-speed defibrator for defibering rejections of the secondary fine screening screen; and n, o and p denote tanks. In FIG. 2, the solid lines represent pulp lines and dotted lines represent rejections including undefibered waste paper.
In FIG. 2, usual screens with hole screen plates are used in the coarse screening stage A. Rejections of the primary screen b is processed by the high-speed defibrator g to defiber undefibered waste paper accumulated in the reject. In the fine screening stage B, a quaternary cascade system with slot screens is used and the rejections of the secondary screen are processed by the high-speed defibrator j.
In FIG. 2, nine apparatuses with screens, seven tanks with agitators and seven pumps are required. For automatic operation, various instruments are further required such as pressure control for each screen and level control for each tank.
Instead of defibering waste paper, the waste paper may be ground by a refiner. Such grinding is, however, directed to crushing not only the undefibered waste paper but also contaminants such as plastics and is different from the defibration in which contaminants such as plastics and wooden pieces are passed without crushing, and therefore has a deteriorated degree of screening compared with the defibration. Also, the stock slurry consistency in the grinding is as high as 15 to 25% while in the defibration, the stock must be diluted to have the consistency of 1 to 4% because of the above difference.
As described above, the more the number of screens for cascade is increased, the more the degree of screening and production yield can be enhanced, but the scale and cost of the facilities are corresponding increased.
To solve the above problems, there have been various proposals to provide a system in which a screening section is combined with a defibering section or with a grinding section.
For example, Japanese Patent 1st Publication No. 62-90391 (JP-A-62-90391) proposes "a screening apparatus with rejection reducing means" which processes pulps with vegetable fiber of 6 to 15% in consistency. A grinding zone is provided adjacent to a screen with a cylindrical screen plate and the reject is decreased in quantity by grinding the rejected stock of the screen into pulpiness. However, when this apparatus is used for waste paper pulp, there arise the following problems:
(1) Unlike vegetable fiber pulp, waste paper pulp includes not only the undefibered waster paper but also contaminants such as plastics and metal pieces. If these contaminants are ground and mingled with the accepted stock, the product quality is decreased. PA0 (2) A consistency suitable for the grinding is 15 to 25%. In the case of waste paper pulp, if the rejected stock of the screen is condensed to this range of consistency, plugging tends to occur in the screen. If the meshes of the screen are enlarged for prevention of such plugging, then the contaminant removal ratio is reduced. PA0 (3) After the grinding, contaminants remain in the pulp. To remove them, another screen is required.
On the other hand, the inventors have made various experiments to find that, when waste paper pulp slurry is screened, rejected stock not passing through a screen are accumulated more and more and its consistency is increased as the slurry flows through a screening section, deteriorating the separation effect, and that the separation effect may be improved if such condensed reject is diluted in the screen.